Agricultural spray equipment normally has used manually actuated valves to turn the spray booms on and off and sprayers of this type usually are pulled by a typical farm tractor. Some may be integrated with specially designed machines which do nothing but spray various chemicals on large acreages. Farm equipment of this general type is constantly being improved and becomes ever more sophisticated. Greater operator comfort is a constant objective and is designed into all new equipment.
Cab equipped spray apparatus have had control valves and related hoses inside the cab and this has created a potentially hazardous situation for the operator since the inside of the cab would be drenched very quickly with the chemicals being sprayed if for any reason the valves or hoses so located should fail. Some of the chemicals used are toxic and could cause severe injury to the operator and to avoid this or reduce the hazard to a minimum the valves and hose connections should be located away from the operator and preferably outside the cab. Solenoid valves offer a convenient way to accomplish this.
However, solenoid valves presently available for use on agricultural sprayers appear to be nothing more than typical industrial valves adapted to the extent necessary for application to the agricultural equipment. The only alternative made in such adaptation usually comprises exchanging the industrial coil for one compatible with automotive type electrical systems. Generally such solenoids are of the normally closed internal pilot operated type and use the system pressure to assist in opening and closing the valve.
Such pilot operated valves typically have a relatively large diameter diaphragm containing two small holes. One hole in the center of the diaphragm is larger than the other which is located nearer the outside perimeter of the diaphragm. A metal armature, which controls the operation of the valve, seats on the center hole and when electrical power is applied the armature moves off of the center hole to allow fluid to flow through the valve. Because the center hole is larger than the outer hole a pressure differential is created across a large area of the diaphragm and the resultant force lifts the diaphragm off of the main seat and allows full flow through the valve. When power is cut off the spring loaded armature is forced to seat on the center hole. The spring force continues to return the diaphragm toward the main seat so that the flow through the valve becomes restricted and a pressure differential is created. This forces the diaphragm into engagement with the main seat thus stopping all flow. Because the armature acts only to control a small area the force created by the coil can by small and therefore a relatively small coil can be utilized to control a large fluid flow.
However, this type of solenoid valve has certain disadvantages. The armature is immersed in the fluid and therefore the armature material must be coated or protected so that it will not be attached by the fluid. The clearances between the armature and the tube in which it moves are small and consequently any insoluble foreign matter such as sand, rust chips, or the like, will cause armature to bind. The fluid velocity on the armature side of the diaphragm is very low and wettable powder solutions commonly used in agricultural applications have a tendency to settle out in this area and restrict the motion of the armature. A minimum pressure differential often is required to open and close the valve fully. A direct acting valve might be used to reduce these problems to a minimum but such valve, having the same flow capabilities as a pilot operated valve, would require a very large coil and the power required for such a coil is so great as to make it impractical due to the fact that sufficient force must be developed to overcome the load created by the pressure acting on the entire seating area.